Solid-state laser device with diffused-light excitation, and integrating sphere

ABSTRACT

In a laser device having a solid-state laser medium of a slab or rod-shaped crystal, a flash lamp serving as an excitation-light source for the laser medium, and a reflector made of a foamed quartz glass for injecting light produced by the light source into the laser medium, an insulator made from a foamed glass is utilized to improve the efficiency of light injection and the resulting beam quality. In addition, where the laser medium is doped with Nd 3+ , a wavelength conversion filter made of a material doped with Cr 3+  or Ti 3+  is utilized to convert most of the excitation light into a wavelength most suitable for exciting the laser medium. Foamed quartz glass is preferred also as the material for the inner surface of an integrating sphere for optical measurement.

This application is a continuation-in-part application of applicationSer. No. 08/228,223 filed on Apr. 15, 1994, U.S. Pat. No. 5,490,161.

BACKGROUND OF THE INVENTION

The present invention relates to solid-state laser devices and tointegrating spheres, and more particularly to reflectors and heatinsulators for solid-state laser devices, an arrangement for providingoptimized excitation light to the solid-state laser medium, andintegrating spheres for use in optical measuring systems.

On account of their compactness and ease of use, solid-state lasers astypified by the YAG laser have been used extensively, especially in thelaser machining field. Recently, solid-state lasers have also found wideapplication in the measurement and medical fields.

Challenges facing the designers of solid-state lasers are raising theoscillation efficiency of the lasers and eliminating undesirable effectcaused by heat build-up in the laser medium. In order to solve theseproblems, the following requirements must be satisfied: (a) the lightemitted by the excitation light source must be efficiently introduced inthe laser medium, and (b) the excitation light must have wavelengthdistribution suitable for laser oscillation.

In a typical solid-state laser device, a laser medium is in the shape ofa rod or a slab (plate), and an electric discharge tube, e.g., a kryptonflash lamp disposed parallel to the laser medium, is used for excitingthe laser medium to emit light for oscillation. For laser efficiency,the manner in which excitation light is injected into the laser mediumis important. For light injection, reflectors made from various highlyreflective materials have been used. Most common among these aregold-plated copper or brass reflectors. Also known are silver-platedreflectors and ceramic reflectors.

A similar need for high reflectance arises with so-called integratingspheres as used in optical measurement systems. If a conventionaloptical detector is used to measure light energy, for example, themeasured value may depend on the size of a light-receiving surface andon the intensity distribution of incident light across thelight-receiving surface. Thus, different beams with the same energy mayproduce different measurement readings. For more consistent readings, anintegrating sphere in which injected light is repeatedly and diffuselyreflected at its inner surface may be utilized so that the intensitydistribution of the light is made uniform. For high reflectance of theinner surface of the integrating sphere, white coatings having MgO orBaSO₄ as their main component are typically used.

Gold-plated reflectors as mentioned above have desirably highreflectance, especially for light of wavelength near 0.8 μm, whichcontributes greatly to the oscillation of YAG lasers, with littledecrease in reflectance of the plated surface due to contamination anddeterioration. However, since there is considerable absorption loss forlight in the 0.5 to 0.6 μm absorption band of Nd:YAG crystals, and thereflectance of gold-plated surfaces falls off at wavelengths below 0.6μm, excitation efficiency of gold-plated reflectors remains limited.Furthermore, because of large absorption in the 0.5 to 0.6 μm wavelengthband, gold-plated reflectors are not suitable for solid-state laserdevices having laser crystals doped with Cr³⁺, such as Cr:Nd:GSGG andCr:BeAl₂ O₂ (alexandrite), as the laser medium.

As reflectors for short-wavelength light, silver-plated reflectors areproblematic because silver forms sulfides having decreased reflectance.This difficulty can be overcome, e.g., by coating the silver surfacewith a protective film of SiO₂ for example, or by silver-plating theback surface of a glass plate so that the silver surface does not comeinto direct contact with cooling water. In high-power lasers, however,silver-plate surfaces are less suitable because of the likelihood ofdamage caused by heat produced by the excitation light.

Ceramic materials have also received recent attention as reflectormaterials for short-wavelength light, but the reflectance of suchmaterials may not always be sufficient. Reflectance may be lower stillwith reflectors in which so-called free-cutting ceramics are used, whichare made by dispersing ceramic particles in a glass matrix.

The above-mentioned problems relating to reflectors for laser devicessimilarly apply to integrating spheres. Although, for weak light, theabove-mentioned coatings containing MgO, BaSO₄ or the like are wellsuited for integrating spheres, at higher powers these coatings'resistance to light becomes unsatisfactory.

In order to achieve improved oscillation with a slab-shaped crystallaser medium, not only the reflector but also the heat insulators forthe crystal must be made of a material which does not absorb light. Inorder to form a one-dimensional thermal gradient, the laser medium of aslab laser has a structure in which a pair of side surfaces of the slabare cooled, between which side surface the laser light propagates alonga zigzag path due to repeated total reflection, and the remaining twoside surfaces are provided with heat insulators to inhibit heat flowtoward them. Since these heat insulators are positioned in the vicinityof the laser medium, their light absorption may cause not only reductionin the efficiency of the laser but also heating of the laser medium dueto heat build-up in the heat insulators. Therefore, heat insulators usedin a slab laser must have high reflectance or transmittance to the laserlight and the excitation light, in addition to having low thermalconductivity. In the past, glass, ceramics and the like have been usedfor such heat insulators.

Another method of improving oscillation efficiency is to convert thewavelength distribution of the excitation light to the distributionsuitable for the excitation of the laser medium. This is based on thefact that only a small range of the spectrum of light emitted by theflash lamp is suitable for laser oscillation, which is a major reasonfor the low efficiency of a lamp-excited solid state laser.

In the above wavelength-conversion approach, the wavelength of theexcitation light is converted by using a substance that emitsfluorescent light having a wavelength suitable for the laseroscillation. One well-known method, as disclosed in Japanese unexaminedpatent publication (Kokai) No. S61-23374, is to use a piece of glassdoped with samarium (Sm) or a piece of glass doped with cerium (Ce) as afilter for excitation light in a solid state laser utilizing a lasermedium doped with Nd³⁺ as an activator. The filter absorbs light in theultraviolet region and emits fluorescent light having a wavelength λ ofabout 0.6 μm. Since Nd³⁺ has one of its absorption peaks in thiswavelength region, the fluorescent light is efficiently absorbed toimprove oscillation efficiency.

A similar method is disclosed in Japanese unexamined patent publication(Kokai) No. H2-123776, which discloses an invention of the presentinventor, in which an AlGaAs semiconductor is used to convert thewavelength of the excitation light. Since the band gap of an AlGaAssemiconductor corresponds to photoenergy of around wavelength λ=810 nm,absorbed excitation light with a wavelength shorter than 810 nm excitescarriers above the conduction band with subsequent recombination thatresults in emission of light of wavelength λ around λ=810 nm as afluorescent light. Since light with 810 nm wavelength is most suitablefor exciting Nd³⁺, highly efficient laser oscillation is achieved forlaser medium doped with Nd³⁺.

In both the method utilizing glass filters doped with Sm or Ce and themethod utilizing AlGaAs semiconductor filters, a part of unutilizedenergy is converted to thermal energy in such filters, which reduces thethermal load to the laser crystal. Therefore, the above-describedfilters are effective means for solving the problems attributable to theheat developed in a solid state laser crystal. However, it has beenknown that the method of providing an excitation light filter with glassbeing doped with Sm or Ce gives low fluorescent emission efficiencywhich only slightly improves the efficiency of a solid state laser.Converting the wavelength of excitation light with an AlGaAssemiconductor is a promising method, but it still has a problem in thatthe refractive index of AlGaAs is as high as 3 or more. If therefractive index is high, most of the excitation light is reflected tobe only a small amount of incident light. In addition, the light whichhas been wavelength-converted therein is not easily emitted to theoutside. Further, this material has a practical difficulty of beingsusceptible to oxidation.

In general, the absorption spectrum of Nd³⁺ doped in a solid state lasermedium exhibits a localized distribution similar to a line spectrum,which poorly matches the spectral distribution of light from a lamp.This is a major cause of low efficiency of a Nd:YAG laser. With respectto improvement of the efficiency of a solid state laser utilizing Nd³⁺as an activator, Koechner describes (W. Koechner, Solid State LaserEngineering, page 57, (3rd ed., Spinger-Verlag Corp., 1992)) a methodutilizing a laser medium doped with multiple elements such as Gd₃ Sc₂Ga₃ O₁₂ doped with Nd and Cr (Nd, Cr:GSGG). Unlike Nd³⁺, Cr³⁺ exhibits aabsorption spectrum widely ranging from 400 to 650 nm. As a result, whena combination of Nd and Cr:GSGG is used, Nd³⁺ is excited by theso-called sensitization, wherein Cr³⁺ absorbs excitation light withwavelengths in a wide range and transfers its energy from the excitedstate of Cr³⁺ to that of Nd³⁺. This results in an improvement inefficiency compared with the case where only Nd³⁺ is used, becauseexcitation light with a wider range of wavelengths is effectively used.In essence, this method changes the absorption spectrum of the lasermedium instead of converting the wavelength of the excitation light.

A method based on a somewhat different idea is disclosed in Japaneseexamined patent publication (Kokoku) No. H5-66035, which discloses asolid state laser rod doped with multiple elements, such as Nd andCr:GSGG, and a Nd:YAG rod arranged in series. The two laser rods aresimultaneously excited by introducing excitation light from the side ofthe multi-element solid state laser rod. The multi-element solid statelaser medium absorbs light with a wide range of wavelengths foroscillation, and light which has passed through the multi-element mediumexcites the Nd:YAG for oscillation. Since the light which has passedthrough the multi-element solid state laser has wavelength distributionsuitable for the excitation of the Nd:YAG, highly efficient laseroscillation can be achieved as a whole.

Although the method utilizing a solid state laser medium doped withmultiple elements such as Nd and Cr:GSGG provides an effective means forimproving efficiency, reduction in the thermal load to the laser mediumcan not be expected because excessive thermal energy is produced in thecrystal. GSGG or the like has a problem in that it is more susceptibleto heat because its thermal conductivity is smaller than that of YAG.

In the method wherein a multi-element solid state laser rod doped withelements such as Nd and Cr:GSGG is placed in series with an Nd:YAG rodand are simultaneously excited by introducing excitation light from theside of the multi-element solid state laser rod, a problem arises inthat laser light is oscillated with two different wavelengths. Althoughthe oscillation of two wavelengths is acceptable in some applications,it results in many difficulties including the fact that the wide rangeof wavelengths increases the effect of chromatic aberration of the lens.This method is also much susceptible to heat, similar to the case inwhich Nd or Cr:GSGG is used alone.

In the context of heat insulators, glass, which has been used in slablasers, absorbs neither excitation light nor laser light. However, glassposes problems in machining and in being insufficiently strong. Ceramicsare relatively well suited as a heat insulator, but they have a problemof being heated by absorbing light in the region of short wavelengths.Further, both glass and ceramics lack sufficiently low thermalconductivity as a heat insulator.

SUMMARY OF THE INVENTION

In a preferred embodiment of the present invention, a solid-state laserdevice has an optical resonator for a solid-state laser medium, a lightsource for exciting the solid-state laser medium, and a reflector forinjecting light from the light source into the solid-state laser medium,with foamed glass as reflector material.

In another preferred embodiment of the present invention, a solid-statelaser device has an optical resonator for a solid-state laser medium, alight source for exciting the solid-state laser medium, and a diffusion(or "diffuse") reflector for injecting light produced by the lightsource into the solid-state laser medium, with a highly reflectivematerial adhered to the back surface of the reflector. The diffusionreflector may be made of foamed glass.

In yet another preferred embodiment of the present invention, anintegrating sphere in which a surface delimiting a spatial region havingat least one light input/output opening is made of a diffuselyreflective material. The diffusely reflective material is foamed glass.The integrating sphere is designed to make the intensity distribution oflight introduced through the input/output opening uniform.

In yet another preferred embodiment of the present invention, a solidstate laser device includes a slab-shaped solid state laser medium witha pair of opposing optically polished side surfaces, a total reflectionmirror and an output mirror arranged at opposite end faces of the lasermedium, and heat insulators made of foamed glass in contact with thesides of the laser medium other than the optically polished surfaces.

In yet another preferred embodiment of the present invention, asolid-state laser device includes an optical resonator for a solid-statelaser medium doped with trivalent neodymium ions, a light source forexciting the solid state laser medium, and a reflector having areflecting surface facing the light source and solid state laser medium.In addition, a wavelength-conversion filter made of a crystal doped withCr³⁺ is interposed between the light source and the solid state lasermedium. In this case, the crystal is preferably one of Y₃ Al₅ O₁₂, Y₃Ga₅ O₁₅, Y₃ Sc₂ Ga₃ O₁₂, Gd₃ Ga₅ O₁₂, Gd₃ Sc₂ Ga₃ O₁₂, La₃ Lu₂ Ga₃ O₁₂,KYnF₃, BeAl₂ O₄, LiSrAlF₆, and LiCaAlF₆.

In yet another preferred embodiment of the present invention, asolid-state laser device includes an optical resonator for a solid-statelaser medium doped with trivalent neodymium ions, a light source forexciting the solid state laser medium, and a reflector having areflecting surface facing the light source and solid state laser medium.In addition, a wavelength-conversion filter made of a glass doped withCr³⁺ or Al₂ O₃ doped with Ti³⁺ is provided between the light source andthe solid state laser medium. The wavelength conversion filters made ofa material doped with trivalent ions of Cr or Ti convert most of theexcitation light to a wavelength most suitable for exciting a lasermedium doped with Nd³⁺. This allows laser oscillation at extremely highefficiency.

It is an object of the invention to provide reflectors for ahigh-efficiency solid-state laser which have high reflectance over awide range of wavelengths extending from the infrared region to theultraviolet region.

It is another object of the present invention to provide heat insulatorsfor slab lasers which facilitate high light-injection efficiency andhigh laser-beam quality without absorbing light over the range from thenear infrared region up to the ultraviolet region.

It is yet another object of the present invention to provide a systemfor obtaining excitation light for solid-state lasers which hasoptimized wavelength for exciting the laser medium.

It is still another object of the present invention to provide anintegrating sphere having an inner surface of high diffusion reflectancewhich can withstand strong light such as a high-power laser light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a longitudinal cross section of a preferred embodiment of aYAG slab laser device according to the present invention.

FIG. 1(b) is a lateral cross section of the YAG slab laser device shownin FIG. 1(a) taken along line 1b--1b.

FIG. 2 is a lateral cross section of an another preferred embodiment ofa YAG slab laser device according to the present invention.

FIG. 3 is a lateral cross section of yet another preferred embodiment ofa YAG slab laser device according to the present invention.

FIG. 4 is a lateral cross section of a preferred embodiment of anintegrating sphere for light measurement according to the presentinvention.

FIG. 5(a) is a longitudinal cross section of still another preferredembodiment of a slab YAG laser device according to the presentinvention.

FIG. 5(b) is a lateral cross section of the slab YAG laser device shownin FIG. 5(a) taken along line 5b--5b.

FIG. 6(a) is a longitudinal cross section of a preferred embodiment of arod-shaped YAG laser device according to the present invention.

FIG. 6(b) is a lateral cross section of the rod-shaped YAG laser deviceshown in FIG. 6(a) taken along line 6b--6b.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in FIGS. 1(a) and 1(b), a laser light 2 propagates through aYAG slab crystal 1 in a zigzag fashion in a first preferred embodimentof the present invention. Shown further are totally reflective mirror 3,an output mirror 4, excitation flash lamps 5, a Pyrex (or borosilicate)glass filter 6 for filtering out ultraviolet rays, an insulatingmaterial 7 on the sides of the YAG slab crystal 1, a reflector 8 madefrom foamed quartz glass, and flows 9 of cooling water. The slab-shapedYAG crystal 1 is disposed between the totally reflective mirror 3 andthe output mirror 4, and the two flash lamps 5 are disposed opposite tothe upper and lower surfaces of the crystal slab 1. The borosilicateglass filters 6 are interposed between the flash lamps 5 and the YAGcrystal slab 1, and the reflectors 8 encircle the flash lamps 5. Theinsulating material 7, which is a foamed quartz glass, contacts thesides of the crystal slab 1 and extend along the length of the crystalslab 1. Since foamed glass, which has numerous internal bubbles, hassmall thermal conductivity, high reflectance and highly malleable,foamed glass is ideally suited to serve as a heat insulator for a slablaser medium. To cool the flash lamps 5, water flows 9 may be channeledthrough cavities defined by the filters 6 and the reflectors 8.

While some of the light produced by the excitation flash lamp 5 passesthrough the Pyrex (borosilicate) glass filter 6 and enters the YAG slabcrystal 1 directly, most of the light follows a more complicated path.Light follows various different paths: some portions of light enter theYAG slab crystal 1 after being irregularly reflected by the foamedquartz glass reflector 8, for example, some portions are absorbed by thecooling water, some escape through the cooling water entrances/exits,some escape through the back surface of the foamed quartz glassreflector 8, and still other portions enter the YAG slab crystal 1 butexit through the opposite side without being absorbed by the crystal.

With a conventional gold-plated reflector, the fraction of lightabsorbed by the reflector may be as much as approximately 50%. A largeamount of excitation light is absorbed by the reflector because, asdescribed above, there are many opportunities for light to be repeatedlyreflected by the reflector, and because the reflectance of gold-platedreflectors drops off at wavelengths less than 0.6 μm. In contrast, inthe preferred embodiment of the invention shown in Figs. 1(a) and 1(b),because absorption in the foamed quartz reflector 8 is low, the portionof light which is absorbed by the YAG slab crystal 1 is increased, andhigh-efficiency laser oscillation is achieved.

In the embodiment shown in FIGS. 1(a) and 1(b), most of the lightproduced by the excitation light source does not directly enter thelaser medium, but enters the foamed quartz reflector and is irregularlyreflected by the minute air bubbles. Of the light that enters thereflector, most is reflected several times and then emitted outside thereflector. Some light is irregularly reflected many times, but, becausethe transmittance of quartz is essentially 100%, sooner or later thislight is emitted to the outside also.

To prevent undue loss of light through the back surface of thereflector, this quartz reflector must have sufficient thickness. Quartzis particularly suited as material for the foamed glass. However, ifsome light absorption at short wavelengths is tolerable, other glassmaterials such as Pyrex can be used instead. Because loss of excitationlight in the reflector is almost eliminated in the first embodiment,high-efficiency laser oscillation becomes possible.

In the embodiment illustrated by FIG. 2, the back surface of a reflector8 made of foamed quartz has a silver-plated surface 10. Some of thelight entering the reflector 8 reaches the back surface of the foamedquartz reflector 8, but, because this light is reflected by thesilver-plate surface 10, light does not escape through the back surface.Thus, the reflectance of the reflector 8 is increased, andhigh-efficiency laser excitation is achieved. Since it is possible tomake the foamed quartz of the reflector 8 thin, the size of the laserdevice can be reduced. As shown in FIG. 2, the reflectors 8 are thinnerthan those shown in FIGS. 1(a) and 1(b).

In the embodiment shown in FIG. 2, unlike the foamed quartz reflector ofthe embodiment shown in FIGS. 1(a) and 1(b) which requires sufficientthickness to prevent light from escaping through to the back surface,the reflectance is raised to prevent light from passing through the backsurface. Accordingly, high reflectance can be realized in a compactreflector for high-efficiency laser oscillation.

For the diffusion reflector, foamed quartz or a free-cutting ceramicmaterial may be used. Also, besides silver as the highly reflectivematerial, other materials can be used including powders of MgO or BaSO₄white coatings used for the inner surface coatings of integratingspheres, and highly reflective resins developed as reflector materialsfor use in lasers.

As mentioned above, when a highly reflective material other than gold isused as a reflector material for a solid-state laser, there have been asevere difficulties due to inadequate resistance to strong excitationlight. With respect to this point, in the second embodiment of FIG. 2,most of the excitation light is reflected by the diffusion reflector,and the amount of light which reaches the highly reflective material onthe rear surface of the reflector is extremely small. Thus, because thehighly reflective material is used only to reflect weak light which haspassed through the diffusion reflector, a reflective material withlesser light resistance can be used. Also, because these highlyreflective materials are adhered to the back surface of the diffusionreflector, without the reflecting surface making contact with coolingwater or the like, materials such as silver can be used where surfacedeterioration would otherwise pose problems.

As shown in FIG. 3, another preferred embodiment of the presentinvention includes magnesium oxide (MgO) powder 12 packed between afoamed quartz reflector 8 and a protective cover 11 which covers theexternal periphery of the reflector 8. Because the reflectance of theMgO powder 12 is high, light is prevented from escaping through the backsurface of the reflector 8 as for the silver plated surface 10 in FIG.2.

It should be apparent that the embodiments shown in FIGS. 2 and 3incorporate reflectors 8 in arrangements which achieve higherreflectance than the embodiment shown in FIGS. 1(a) and 1(b).Accordingly, the embodiments shown in FIGS. 2 and 3 achieve moreefficient laser oscillation. Furthermore, in the embodiments shown inFIGS. 2 and 3, the foamed quartz reflector 8 can be made thinner thanthat in the embodiment shown in FIG. 1(a), thereby achieving increasedcompactness of the laser device.

Shown in FIG. 4 is an integrating sphere 13 with an interior spatialregion 14, an incident opening 15 for laser light 16 to be measured, anemergent opening 17, and a light detector element 18 disposed in theemergent opening 17. The body of the integrating sphere 13 is made offoamed quartz. Laser light 16 enters the spherical space 14 in theintegrating sphere 13 through the incident opening 15 and reaches theemergent opening 17 after being repeatedly and irregularly reflected bythe inner surface of the spherical space 14. The intensity of the lightin the emergent opening 17 is then detected by the detector element 18.

In the integrating sphere shown in FIG. 4, the foamed quartz glass,because of its high light-reflectance, repeatedly and irregularlyreflects the laser light 16 entering the integrating sphere, thus makingthe intensity distribution of the light uniform. Since the intensitydistribution of the laser light entering the sphere does not affectmeasurement, high-precision measurements are possible. Furthermore,because the resistance of the foamed quartz to laser light is high, thisintegrating sphere can be used safely with high-power lasers.

Shown in FIGS. 5(a) and 5(b) is another embodiment of the solid-statelaser device according to the present invention. In this embodiment,wavelength conversion filters 51 are used instead of the borosilicateglass filters. The wavelength conversion filters 51 convert light fromthe flash lamps 41 into light of wavelength λ around λ=810 nm, which ismost suitable for the excitation of YAG crystal 1. Material for thefilters 51 may be any one of Y₃ Al₅ O₁₂ (YAG) , Y₃ Ga₅ O₁₅ (YGG), Y₃ Sc₂Ga₃ O₁₂ (YSGG) , Gd₃ Ga₅ O₁₂ (GGG), Gd₃ Sc₂ Ga₃ O₁₂ (GSGG), La₃ Lu₂ Ga₃O₁₂ (LLGG), KYnF₃ (KYF), BeAl₂ O₄ (alexandrite), LiSrAlF₆ (LiSAF), andLiCaAlF₆ (LiCAF), all of which are doped with Cr³⁺. The filters 51 mayalso be glass doped with Cr³⁺, or Al₂ O₃ (sapphire) doped with Ti⁺³. Inorder to prevent these materials from deteriorating under ultravioletrays, however, the excitation flash lamps 41 are doped with cerium (Ce).

The above-listed materials used as the wavelength conversion filters 51have been previously used as solid-state laser media. However, becausethe emission spectra of these materials have their peaks at thewavelength λ in the vicinity of λ=810 nm, which is suitable for theexcitation of Nd³⁺ as described in, for example, Japanese unexaminedpatent publication (Kokai) No. H2-123776. Accordingly, these materialsare well suited for converting the wavelength of excitation light.

When the above-listed materials for the wavelength conversion filters 51are used as a laser medium, they must have high optical uniformitythroughout the crystal. This poses a significant problem because of thedifficulty of growing a large crystal that satisfies such a requirement.However, for the purpose of functioning as a filter for converting thewavelength of excitation light, such a high optical uniformitythroughout the crystal is not required, and a large crystal for thispurpose can be easily produced. The function of serving as a wavelengthconversion filter will be sufficiently accomplished even by a crystalwhose thermal conductivity is so small as to cause a strong thermal lenseffect, which would disqualify the crystal to function as a lasercrystal.

For example, a crystal such as Al₂ O₃ doped with Ti³⁺ (Ti:sapphire) mustbe strongly excited to be used as a laser crystal because the durationof fluorescence is short. However, the short duration of fluorescencemeans the high transition probability for the high fluorescent emissionefficiency. This is rather preferable for a wavelength conversionfilter. Such wavelength conversion filters absorb excitation light ofwavelength λ=400 to 650 nm and emit light of wavelength λ around λ=810nm which is suitable for the excitation of Nd³⁺. Since the light ofwavelength λ around λ=810 nm is of course transmitted through the filterwithout absorption, the total efficiency is improved by an amountcorresponding to the portion of excitation light which has beensubjected to wavelength conversion.

In the embodiment shown in FIGS. 5(a) and 5(b), amongst the beams oflight emitted by the flash lamps 41, beams of wavelength λ around λ=810nm are transmitted through the wavelength conversion filters 51 and aredirectly incident on the YAG crystal 1. Contrastingly, most of the beamsof wavelength λ=400 to 650 nm are absorbed by the wavelength conversionfilters 51 and converted to beams of wavelength λ around λ=810 nm to beabsorbed by the YAG crystal 1. Accordingly, most of the excitation lightemitted by the flash lamps 41 is converted to that of wavelength λ atλ=810 nm, which is most suitable for the excitation of the YAG crystal1, and this allows extremely efficient laser oscillation. In addition,most of optical energy which does not contribute to laser oscillation isconverted into thermal energy in the wavelength conversion filters 51.As a result, the thermal load of the YAG crystal and the negativeeffects attributable to temperature rise on the crystal, such as athermal lens effect, are reduced, thereby improving the quality of theoscillated laser beam.

Shown in FIGS. 6(a) and 6(b) is an embodiment of a rod-type YAG laserdevice according to the present invention. In this embodiment, the lasermedium is a rod-shaped Nd:YAG crystal 21, and a reflector 22 made ofceramic is used. The reflector 22 surrounds a tubular wavelengthconversion filter 52, which in turn surrounds the rod-shaped crystal 21,and a flow tube 23 made of borosilicate glass. The flow tube 23, whichin turn surrounds a flash lamp 105, serves as a channel for coolingwater which flows from an inlet 24 to an outlet 25 for cooling the flashlamp 105. A space between the YAG crystal rod 21 and the filter 52serves as a channel for cooling water which flows from an inlet 26 to anoutlet 27 for cooling the YAG crystal rod 21. A space 28 defined by thefilter 52, flow tube 23, and the reflector 22 is also filled withcooling water. The material for the filter 52 may be any of Cr:YAG,Cr:YGG, Cr:YSGS, Cr:GGG, Cr:GSGG, Cr:LLGG, Cr:KYF, Cr:LiSAF, Cr:LiCAF,Cr-doped alexandrite, Cr-doped glass, and Ti-doped sapphire.

In the preferred embodiments, the foamed glass is a quartz glass, SiO₂,containing small bubbles of a few hundred microns in size, with specificgravity of one-tenth to one-half of ordinary quartz glass. Since thismaterial has low thermal conductivity and is easily machinable, it canbe used for thermal insulation, for example. The thermal conductivity offoamed quartz glass is as low as one tenth of the thermal conductivityof ordinary quartz glass. Although ordinary quartz glass has hightransmittance at wavelengths down to the ultraviolet region, foamedquartz glass has high reflectance due to the many internal solid-to-gasinterfaces. This may be likened to the high reflectance of snow which isan aggregate of fine ice particles.

I claim:
 1. A solid-state laser device comprising:a solid-state lasermedium having a pair of opposing surfaces, a first longitudinal end, anda second longitudinal end; a totally reflecting mirror disposed at saidfirst longitudinal end of the laser medium and a partially transparentmirror disposed at said second longitudinal end of the laser medium; alight source for exciting said solid-state laser medium; means forintroducing light produced by said light source into said solid-statelaser medium, said light introducing means comprising a glass reflectorhaving a plurality of bubbles therein; and means, coupled to said pairof opposing surfaces of said laser medium, for insulating said pair ofopposing surfaces, said insulation means comprising a glass having aplurality of bubbles therein.
 2. The solid-state laser device of claim1, wherein the solid-state laser medium consists essentially of YAGcrystal.
 3. The solid-state laser device of claim 1, wherein the glassreflector consists essentially of quartz glass having a plurality ofbubbles therein.
 4. The solid-state laser device of claim 1, wherein theglass insulation means consists essentially of quartz glass having aplurality of bubbles therein.
 5. The solid-stage laser device of claim1, further comprising a wavelength filter means disposed between thelight source and the laser medium, said wavelength filter meansfiltering ultraviolet rays.
 6. The solid-state laser device of claim 5,wherein the wavelength filter means consists essentially of borosilicateglass.
 7. The solid-state laser device of claim 1, wherein the lightsource comprises a krypton flash lamp.
 8. The solid-state laser deviceof claim 3, wherein said glass reflector has an outer surface, saiddevice further comprising a packed powder insulation layer coupled tosaid outer surface of said glass reflector, said powder being selectedfrom the group consisting essentially of magnesium oxide and bariumsulfate.